Liquid‐Phase Cyclohexene Oxidation with O2 over Spray‐Flame‐Synthesized La1−x Sr x CoO3 Perovskite Nanoparticles

Abstract La1−x Sr x CoO3 (x=0, 0.1, 0.2, 0.3, 0.4) nanoparticles were prepared by spray‐flame synthesis and applied in the liquid‐phase oxidation of cyclohexene with molecular O2 as oxidant under mild conditions. The catalysts were systematically characterized by state‐of‐the‐art techniques. With increasing Sr content, the concentration of surface oxygen vacancy defects increases, which is beneficial for cyclohexene oxidation, but the surface concentration of less active Co2+ was also increased. However, Co2+ cations have a superior activity towards peroxide decomposition, which also plays an important role in cyclohexene oxidation. A Sr doping of 20 at. % was found to be the optimum in terms of activity and product selectivity. The catalyst also showed excellent reusability over three catalytic runs; this can be attributed to its highly stable particle size and morphology. Kinetic investigations revealed first‐order reaction kinetics for temperatures between 60 and 100 °C and an apparent activation energy of 68 kJ mol−1 for cyclohexene oxidation. Moreover, the reaction was not affected by the applied O2 pressure in the range from 10 to 20 bar. In situ attenuated total reflection infrared spectroscopy was used to monitor the conversion of cyclohexene and the formation of reaction products including the key intermediate cyclohex‐2‐ene‐1‐hydroperoxide; spin trap electron paramagnetic resonance spectroscopy provided strong evidence for a radical reaction pathway by identifying the cyclohexenyl alkoxyl radical.

. Williamson-Hall plots of La1-xSrxCoO3 samples. B indicates the integral breadth. The measured lattice strain values for LCO, Sr10, Sr20, Sr30 and Sr40 are 0.0107, 0.0068, 0.0059, 0.0043 and 0.0028, respectively.   Figure S3. TEM images showing the particle size distributions and the histograms fitted with lognormal function in order to determine dp and σg.      The surface chemical groups in the C 1s and O 1s spectra were investigated ( Figure S6 and S7). The oxygen-bound carbon groups appear in the C 1s spectra at 290-286 eV, being related to carbonaceous combustion residuals remaining in the nanoparticles. Slightly higher peaks of carbonate and carboxylate groups are visible in the XP spectrum of Sr20 compared with the other samples ( Figure  S6a). On the other hand, regarding the overall content of carbonaceous residuals, ATR-FTIR spectra in Figure S6b show that the intensities of C-O, C=O, O=C-O, CO3 groups relative to those of Co ̶ O are higher in Sr20, Sr30 and Sr40, indicating higher amounts of bulk carbon species for high Sr concentrations. Additionally, Figure S7 shows the deconvoluted O 1s XP peaks with the lattice oxygen species of the perovskite at 528.6 eV as well as Osurface (O2 2-/O -) at 529.5 eV, Ocarbon/OH at 531.5 eV and Owater at 533.0 eV. Ocarbon/OH intensities show an increase for Sr20, Sr30 and Sr40 in agreement with the band intensity of carbon-oxygen groups in the ATR-FTIR spectra. These peaks mainly correspond to Ocarbon species, as OH groups were not detected in the ATR-FTIR spectra. In addition, the O2 2-/Opeak intensity increases for Sr30 and Sr40, which can be related to oxygen vacancies in the perovskite materials [61] as well as oxygen species of Co3O4.
Figure S13. Correlations of cyclohexene conversion (above) and hydroperoxide selectivity (below) after 6 h with specific surface area (left) and particle size (right) of the La1-xSrxCoO3 perovskites.       Figure S21. ATR-IR spectra of standard compounds in acetonitrile.  Figure S22. Untreated in situ ATR-IR spectra for the oxidation of cyclohexene over Sr40 catalyst in the range of 1800 to 650 cm -1 after background subtraction. The spectrum for background subtraction is taken of the mixture of acetonitrile, o-dichlorobenzene as internal standard for GC analysis, cyclohexene and Sr40 catalyst under N2 atmosphere at 80 °C.

Catalyst Synthesis
Spray-flame synthesis of perovskite nanoparticles was performed in a homemade reactor which allows full control of gas flow and pressure. The reactor setup was described previously, [24c,26a,52] and its scheme is shown in Figure S24. The process parameters and the precursor/solvent compositions used for the syntheses are tabulated in Table S4. In brief, propionic acid was mixed with 1-propanol in a glass flask at room temperature and stirred for 5 min. The La-, Co-, and Sr-precursors were subsequently added to the solvent mixture and the flask was heated in a silicon oil bath at 60 °C to dissolve the precursors under constant magnetic stirring for at least 1 h. The total metal concentration of (La+Sr) and Co was adjusted to 0.2 M. The nominal atomic ratios of Sr/(La+Sr) (xSr) were adjusted to 0, 0.1, 0.2, 0.3, and 0.4. Afterwards, the precursor solution was transferred into the reactor where the dispersion O2 gas atomizes the precursor solution. For LaCoO3, La0.9Sr0.1CoO3 and La0.8Sr0.2CoO3, the dispersion O2 gas of 6 slm with a precursor flow rate of 3 mL/min was used. For La0.7Sr0.3CoO3 and La0.6Sr0.4CoO3, the gas to liquid volume ratios was reduced by the dispersion O2 gas of 7 slm with a precursor flow rate of 4 mL/min being used. Premixed pilot flame (CH4/O2) surrounding the hollow needle was used to ignite and stabilize the spray flame while a sheath gas surrounding the flame was used to stabilize the reactor gas flow ( Figure S24). The reactor was operated at a pressure of 925-930 mbar. The particles formed were collected on a circular filter membrane from the burner off-gases.

X Ray Diffraction (XRD) Measurements
X-ray diffraction (XRD) patterns were measured with a PANalytical X'Pert PRO X-ray diffractometer operated at 40 kV and 40 mA using Ni-filtered Cu-Kα radiation and a linear position sensitive X'Celerator detector in combination with a monochromator. A silicon single crystal was used as sample holder and the diffraction data of as-prepared nanoparticles were measured in the 2θ range from 10-65° with a step size of 0.03° and a scan speed of 0.05°/min. The average crystal size and microstrain of the nanoparticles was estimated by the Williamson-Hall method. As octahedral-shaped crystals were mostly observed by TEM measurements, the Scherrer constant of octahedral crystallites (i.e., Kβ ~ 1.1 [53] ) was used as shape factor.

N2 Physisorption
N2 physisorption measurements were performed at 77 K in a Quantachrome instruments apparatus. The asprepared powders were firstly degassed at 100 °C under vacuum for 5 h to remove adsorbed water. The specific surface areas were determined from the adsorption isotherm using the BET method. The bulk density values were taken from ICSD data files of La1 ̶ xSrxCoO3.

High Resolution Transmission Electron Microscopy (HR TEM)
High-resolution and high angle annular dark-field scanning transmission electron microscopy (HR-TEM, HAADF-STEM), and energy dispersive X ray spectroscopy (EDX) were performed at a JEM-2200FS (JEOL, Akishima, Japan). For sample preparation, nanoparticles were dispersed in ethanol for 5 min with ultrasonic treatment, and the dispersions of the nanoparticles were dropped on a carbon-coated copper grid for TEM measurements. For estimating count mean particle diameters (CMDs), the diagonal distance of 400 different particles per sample were measured. The size histograms were fitted with lognormal curves to determine CMDs.

X Ray Photoelectron Spectroscopy (XPS)
VersaProbe II (Ulvac-Phi, Chanhassen, USA) was used to measure XP spectra of the nanoparticles. Al Kα radiation (hv= 1253.6 eV) with a pass energy of 11.75 eV was used to obtain the Co 2p, La 3d, Sr 3d, C 1s, and O 1s XP spectra. The binding energies of all spectra were corrected to C 1s at 284.8 eV. For the deconvolution of Co 2p and Sr 3d spectra, the binding energies and FWHM reported in the literature were taken into account. [32a,54] Deconvolution of O 1s spectra was carried out referring to a previous report. [36] The deconvolution of C 1s spectra and the calculation of carbon-bound oxygen intensity in the O 1s spectra were realized using the method described in the literature. [55] Superconducting Quantum Interference Device (SQUID) Magnetometry The temperature-dependent magnetization of La1-xSrxCoO3 was measured from powder samples of solid material immobilized in eicosane by using a superconducting quantum interference device (SQUID, MPMS-7, Quantum Design, calibrated with a standard palladium reference sample, error < 2 %) with a field of 50 mT (field cooled samples) or 0 T (zero field cooled) in the temperature range from 2 to 300 K.
Gas-Phase Oxidation of 2-Propanol 2-Propanol oxidation was carried out in a stainless steel microreactor setup. 10 mg catalyst powders of the sieve fraction 250-355 µm were filled in a glass lined stainless steel U-tube reactor. To minimize the effect of hot spots, the samples were diluted with 50 mg quartz pearls of the same size. A thermocouple was directly placed into the catalyst bed to control the temperature. Passing He through a saturator at 273 K enabled 2-propanol dosing. By using mixing valves, 2-propanol/He and O2 were mixed and diluted with He. For 2-propanol oxidation, 0.18 % 2-propanol/ 0.18 % O2/ He feed gas was passed over the catalyst for 1 h and the sample was heated to 573 K with a heating rate of 0.5 K min -1 . A calibrated quadrupole mass spectrometer (QMS, Balzers GAM422) was used for time-resolved quantitative online gas analysis.

Catalytic Oxidation Reactions
Oxidation reactions were carried out in a 100 mL autoclave reactor equipped with a Teflon liner (Parr Instruments). 60 mg catalysts were dispersed in 30 mL acetonitrile. 20 mmol cyclohexene and 4.5 mmol 1,2-dichlorobenzene as the internal standard for GC analysis were added. The autoclave was purged with O2 for three times and pressurized to 10 bar. Subsequently, the reaction mixture was heated to 80 °C. The reaction was initiated by switching on the stirrer to 600 rpm at 75 °C. Equally, stirring was started 5 °C below the set temperature for oxidation reactions performed at different temperatures. Samples were taken through an online sampling system after 1, 2, 4, and 6 h. For GC analysis, two samples of 1.5 mL were taken after filtering off the catalyst by a syringe filter (200 nm). One sample was purely analyzed while the other was treated with 2 mmol triphenylphosphine (PPh3) to decompose the formed 2-cyclohexene-1-hydroperoxide into the corresponding alcohol 2-cyclohexene-1-ol. The hydroperoxide amount was calculated by subtraction of the detected alcohol amounts. The errors of the individual measurements were calculated by using the Gauss law for propagation of error considering the standard deviations of the GC measurements and of the calibration.

Catalyst Reusability
To test the reusability of the catalyst, three reaction runs were carried out under standard conditions. After each run, the catalyst was separated by centrifugation, washed three times with 5 mL acetonitrile, and dried overnight at room temperature. The catalyst amount decreased from initially 60 mg, to 56 and 51 mg in the second and third run, respectively.
EPR Spectroscopy X-band cw EPR spectra were obtained using an X-band Bruker Elexsys E500 EPR spectrometer equipped with a ER4116DM dual mode resonator and an ESR 900 He cryostat. La1-xSrxCoO3 were measured as powder samples (~3 mg) for detection of Co 2+ . The EPR spectra were obtained at 10 K using a microwave frequency of ~9.

ATR-IR Spectroscopy
ATR-IR spectra were recorded using a Thermo Scientific 6700 FTIR spectrometer. A home-made ATR cell described elsewhere [57] was used with a Ge IRE (Korth Kristalle GmbH). 10 mg Sr20 was suspended in 1 mL distilled water and sonicated for 15 min. The suspended sample was deposited on the IRE by drop coating and the solvent was evaporated by heating to 80 °C. The deposit-evaporate procedure was repeated. Spectra were recorded with 256 scans in the range from 4000 -500 cm -1 and a resolution of 4 cm -1 . The ATR cell was heated to 60 °C during the measurements. The catalyst film in contact with 1 atm air was used as background. For the measurement, acetonitrile was filled into the ATR cell and spectra were recorded. Afterwards, cyclohexene (0.67 mol L -1 ) was added and further spectra were recorded. Reference spectra of acetonitrile and cyclohexene were obtained according to the experimental procedure by using an uncoated Ge IRE.
In situ ATR-IR spectroscopy was carried out with a ReactIR 15 from Mettler Toledo equipped with a diamond internal reflection element (IRE), an AgX 6 mm x 1.5 m fiber probe, and a liquid nitrogen-cooled mercurycadmium-tellurid (MCT) detector. The spectral range of 3000 to 650 cm -1 was recorded with a resolution of 4 cm -1 . For the in situ ATR-IR study, 200 mg Sr40 were suspended in 150 mL acetonitrile. 100 mmol cyclohexene and 22.5 mmol 1,2-dichlorobenzene as internal standard for GC analysis were added. The reaction solution was filled in a Teflon liner of a 350 mL autoclave (Berghof) equipped with an ATR-IR probe (Mettler Toledo). The reactor was flushed three times with nitrogen and pressurized to 3 bar. It was heated in the inert atmosphere to 80 °C under stirring at 300 rpm. After the temperature was stabilized, the oxidation reaction was initiated by pressurizing the reactor with oxygen to 20 bar. The reaction progress was monitored by recording the spectra every 2 min. Meanwhile, samples were taken through an online sampling system after 0, 1, 2, 3, and 4 h and anayzed by GC.

Gas chromatographie
Gas chromatography analysis was carried out in a 7820 A GC from Agilent Technologies. It was equipped with an Agilent DB-XLB column (30 m x 0.18 mm x 0.18 μm) and an FID detector. The injection volume was set to 0.5 μL with a split ratio of 75:1, a split flow of 30 mL/min and an inlet temperature of 260 °C. The column was first kept at 80 °C for 5 min. Subsequently, the oven was heated to 170 °C with a rate of 15 °C/min. Afterwards, it was heated with a ramp of 30 °C/min up to 300 °C to avoid deposits of the PPh3 in the column. The end temperature was kept for 1 min.